feat(crumb): Configure Firefox

- Set search engines.
- Install extensions
- Set basic Firefox settings.
- Bump home-manager to use updated `programs.firefox` API.  Namely,
  `programs.firefox.profiles.NAME.extensions`.

flake.nix

@@ -12,6 +12,11 @@
     sops-nix.url = "github:Mic92/sops-nix";
     sydnix-cli.url = "path:///persist/dots/scripts/sydnix-cli";
     copyparty.url = "github:9001/copyparty";
+    # Used for Firefox extensions/addons.
+    nur = {
+      url = "github:nix-community/NUR";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs = { nixpkgs, ... }@inputs: